Providing a vent hole on an airbag cushion and allowing gas to flow out from the interior of the airbag cushion through the vent hole when a vehicle occupant or the like plows into the inflated airbag cushion to receive and constrain the vehicle occupant or the like softly by the airbag cushion is publicly known.
In Japanese Unexamined Patent Application Publication No. 2005-199987, an airbag cushion in which the vent hole is covered with the lid member from the inside of the airbag cushion is described. In the same publication, a proximal end portion of the lid member on the side of the inflator, that is, on the side of the center of the airbag cushion with respect to the vent hole is inseparably joined to the airbag cushion. Also, a distal end portion of the lid member on the opposite side from the inflator with the intermediary of the vent hole, that is, on the side of the outer periphery of the airbag cushion is placed between a front surface, that is, an occupant-faced surface and a rear surface of the airbag cushion at an outer peripheral portion of the airbag cushion and these members are releasably sutured by a sewing yarn which is broken when the interior of the airbag cushion reaches or exceeds a predetermined pressure, so-called a tear seam integrally.
In the Patent Application Publication described above, the outer peripheral portions of the front surface and the rear surface of the airbag cushion are sutured by the teat seam over the entire periphery of the airbag cushion.
In the airbag cushion of the Patent Application Publication described above, in an initial stage of inflation of the airbag cushion by gas from the inflator, the joint among the front surface and the rear surface and the lid member of the airbag cushion by the tear seam is not released, so that the lid member assumes a state of being overlapped in the vent hole. Accordingly, the flow out of the gas from the vent hole is restrained and the interior of the airbag cushion is rapidly brought into a high pressure, whereby the airbag cushion is rapidly inflated.
Then, when the pressure in the interior of the airbag cushion exceeds a predetermined pressure, the tear seam is broken, the front surface and the rear surface of the airbag cushion are moved apart from each other, and the joint between them and the lid member is released, whereby the lid member is pushed out from the airbag cushion through the vent hole by the gas pressure in the interior of the airbag cushion. Accordingly, the vent hole is opened, and when a vehicle occupant is brought into contact with the inflated airbag cushion, the gas is flowed out from the interior of the airbag cushion through the vent hole, so that the vehicle occupant is softly received by the airbag cushion. Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-199987
According to the airbag cushion in the Japanese Unexamined Patent Application Publication No. 2005-199987 described above, the outer peripheral portions of the front surface and the rear surface of the airbag cushion are sutured over the entire periphery by the tear seam. Therefore, it takes time until the entire tear seam is broken.
In addition, when a portion of the tear seam suturing the front surface, the rear surface and a distal end portion of the lid member is broken at a timing earlier than other portions, the vent hole is opened and gas is flowed out from the airbag cushion before the entire tear seam is broken. In this case, the breakage of the tear seam may be delayed, or the tear seam remains partly unbroken and the entire airbag cushion can hardly be inflated uniformly.